When innovation is cumulative, patent protection on early inventions can generate holdup problems if later complementary patents are owned by different firms. We show that shareholder ownership overlap across firms with patent complementarities helps mitigate such holdup problems and correlates significantly with higher R&D investment, more patent success, and lower patent infringement litigation risk for firms with follow-on innovations. The positive innovation effect is stronger for concentrated overlapping ownership and when overlapping shareholders are dedicated investors, with long investment horizons and underdiversified portfolios.